The present invention relates to an output circuit which is provided between the output of a first circuit and the input of a second circuit formed in an integrated circuit such as a semiconductor integrated circuit, and which outputs a signal of a level based on a signal supplied from the first circuit to the second circuit. Such an output circuit is usually provided in an integrated circuit. In this case, the second circuit is referred to as an external circuit or an external interface.
FIGS. 9 through 11 are circuit diagrams illustrating conventional output circuits employed for MOS integrated circuits. FIG. 9 shows a CMOS output circuit in which an output section is configured using a CMOS circuit composed of a pMOS transistor 103 and an nMOS transistor 104. FIG. 10 shows an nMOS open-drain type output circuit which employs the nMOS transistor 104 having an open drain electrode. FIG. 11 shows an nMOS pull-up drain type (nMOS open-drain type with a built-in pull-up device) output circuit in which an output section is composed of a pMOS transistor 302, which stays ON constantly, and an nMOS transistor 104 having the drain electrode thereof pulled up by the pMOS transistor 302. There are also available a pMOS open-drain type output circuit and a pMOS pull-down drain type (pMOS open-drain type with a built-in pull-up device) output circuit. The nMOS pull-up drain type and the pMOS pull-down drain type are referred to simply as pull-drain type. In FIG. 9 through FIG. 11, an input signal IN is applied to an input terminal xe2x80x9cinxe2x80x9d and transmitted to an input signal conductor 10 via an inverter 101. An output signal OUT is output through an output terminal xe2x80x9cout.xe2x80x9d
The conventional output circuits set forth above are provided with signal output types, namely, CMOS type, open-drain type, and pull-drain type, and a driving capability to comply with the specifications of an external interface to be connected thereto. This has been posing a problem in that, when the external interface connected thereto is changed, the output circuit that has been used before the change cannot be used as it is, meaning that the internal circuitry of the output circuit has to be changed. There has been another problem in that, when two or more external interfaces are expected to be used, the same number of output circuits compatible with the respective external interfaces as that of two or more external interfaces must be prepared in advance.
The present invention has been made with a view toward solving the problems described above. It is an object of the present invention to provide an output circuit that can be applied to two or more external interfaces.
The output circuit according to the present invention has an input circuit generating an internal input signal in response to an external input signal, an output circuit including a pMOS transistor coupled between a positive potential source and an output terminal and an nMOS transistor coupled between a reference potential source and the output terminal. A gate of one of the transistors receives the internal input signal. The output circuit further has a control circuit receiving the internal input signal and the control signal and outputting an internal signal in response to the internal input signal and the control signal to a gate of the other one of the transistor. The control circuit outputs the internal input signal as the internal signal where the control signal has a first level. The control circuit outputs a predetermined level signal as the internal signal where the control signal has a second level so that the other one of the transistor has a predetermined state.